The average consumption of power in a typical Australian community is about 1 kWhr per hour per person. The production of sewage is about 200 litres per day per person. At present, the provision of electricity is not seen as being complementary to, or logically associated with, the treatment and possible re-use of sewage.
Treated sewage, in many instances, is simply discharged into the sea or into inland river systems and the enhanced nitrogen and phosphorus content of sewage, as opposed to the raw water supplied to households, is associated with the production of algal xe2x80x9cbloomsxe2x80x9d in rivers and waterways. This has resulted in attention being focussed on reducing the nitrogen and phosphorus content of treated sewage. Reductions are of course possible with complex sewage treatment and/or including processing operations such as the addition of chemicals and further settling, biological processes, filtration operations and the like.
It is possible to use treated sewage with a high nitrogen and phosphorus content for irrigation, and in the production of crops, fodder and the growth of trees, lawns, gardens and the like, effective use being made of the nitrogen and phosphorus content and the water run off having reduced levels of nitrogen and phosphorus. However a problem with the use of treated sewage for irrigation is that the colliform and pathogen content can typically be about 100-200 units per 100 ml. This can result in a number of problems and disadvantages (such as a significant health risk, the relocation of people during and for a period after the application of treated sewage, the non use of resultant agricultural produce for human consumption unless the end product is cooked under controlled conditions). For these reasons treated sewage cannot be generally used for normal agriculture, accessible parklands, external washing purposes or as a household xe2x80x9cgreyxe2x80x9d water.
Further, in some locations in NSW for example, whilst there is a demand for approval of building lots for further construction of houses, such approvals are being impeded by the effluent problems associated with conventional sewage treatment plants. There is also the perceived need to preserve and enhance the regions near inland creeks and waterways so as to provide visually attractive xe2x80x9cgreen beltsxe2x80x9d and recreational areas which, under normal conditions, require the use of substantial amounts of water for strong plant and tree growth.
It is known that the term sewage may also incorporate various industrial effluents, food processing wastes, agricultural wastes and other waste water streams which may or may not contain pathogens and the like but which may be mixed with household type sewage. Again, this cannot be used with safety.
It is also known that the anaerobic digestion of sewage converts sewage organic matter to methane and carbon dioxide. However, due to its high carbon dioxide content and low pressure this gas can be expensive to compress to enable its use in gas turbines. Anaerobic digestion requires heat which can be costly.
A still further problem with sewage plants is the associated gas and odour releases. It is known that sewage sludge produced in sewage treatment decomposes to produce methane and foul smelling sulphur-containing gases such as hydrogen sulphide. There is a need to eliminate the foul odours and if possible utilize these gases.
A further general problem with population growth and new housing is the recognised need to reduce greenhouse gas emissions, which are predominantly associated with the production of electricity. In recent years the trend to install large coal-fired power stations remote from populous areas has been the preferred mode of supplying power (e.g., in NSW). However, this system has the problem of creating substantial greenhouse gas emissions compared to gas-fuelled power plant and the need for high tension transmission corridors which are now seen as unsightly and a potential source of electromagnetic radiation associated with unquantifiable health risks. A further problem with coal field-located coal-fired power plant, and long high voltage transmission systems, is the appreciable amount of power lost in electricity transmission.
Whilst changing from remote coal-fired power plant to gas-fired plant has the desired benefit of reducing carbon dioxide emissions associated with power generation, the generation of gas-based power in populous areas has the disadvantage of discharging nitric oxide (NOX) emissions from power generation in such areas (it is known that NOX emissions are a precursor to a range of deleterious tropospheric reactions such as ozone and smog formation). In many populous areas strict controls are placed on power generation plant with a best achievable NOX content of the exhaust from gas turbines being, typically, about 20 ppm. This may be regarded as being acceptable by some authorities. However there is a perceived need, and a known problem, in lowering NOX emissions and a major problem in being able to lower emissions in gas turbine exhausts below 5 ppm. For example, Los Angeles, which is known to have severe tropospheric pollution problems, has a current requirement for NOX discharges in gas turbines to be less than 5 ppm. At present this level of NOX emissions cannot be achieved with commercially available gas turbines.
By means of this invention the described problems associated with the disposal of sewage and treated sewage, and the generation of power are addressed and, at least in part, are solved.
Accordingly, in one aspect, the invention provides a method for the generation of electricity and the complementary purification of sewage characterized in that:
(I) exhaust gases, from a mixture of compressed air or compressed air and contaminated air and fuel gas combusted in a gas turbine for the generation of electricity, are utilized in a first step of heating and disinfecting a stream of sewage from a sewage treatment plant; and
(ii) the thus disinfected hot sewage stream is contacted with, and saturates, a stream of the compressed air or compressed air and contaminated air prior to its admixture with the fuel gas.
In a further aspect, the invention provides a method for the production of electricity and the complementary purification of sewage comprising the following steps:
(I) introducing, into a heating vessel, a stream of sewage from a sewage treatment plant;
(ii) introducing, into the compressor of a gas turbine, a stream of air or optionally a stream of air and contaminated air;
(iii) conveying the compressed air or compressed air and contaminated air to a mixing device wherein the compressed air/contaminated air is mixed with a stream of fuel gas separately introduced at the mixing device;
(iv) combusting the mixture of compressed air/contaminated air and fuel gas to produce hot gases which are fed to the expansion stage of the turbine which drives an alternator to produce electricity;
(v) conveying exhaust gases from the expansion stage of the turbine to the heating vessel containing the introduced stream of sewage, the sewage being heated to a temperature, and retained at the temperature for a period of time, sufficient to disinfect the sewage and thus ensure that all pathogens and bacteria in the sewage are effectively destroyed;
(vi) introducing the thus disinfected hot sewage to a pressure vessel where it is brought into contact with, and saturates, the stream of compressed air or compressed air/contaminated air being conveyed from the compressor to the mixing device; and
(vii) discharging the thus purified and disinfected sewage for use as required.
There is also provided, in accordance with the invention, apparatus for the generation of electricity and the complementary purification of sewage comprising in combination:
(I) a heating vessel;
(ii) means for introducing a stream of treated sewage to the heating vessel;
(iii) means for heating the treated sewage in the heating vessel to disinfect the treated sewage, the heating means comprising the exhaust gases from a mixture of separately introduced compressed air or compressed air and contaminated air and fuel gas combusted in a gas turbine to generate electricity;
(iv) means for conveying the introduced pre-mixed compressed air/contaminated air to a device where it is mixed with the introduced fuel gas;
(v) means for introducing the hot disinfected sewage into the compressed air/contaminated air conveying means, the disinfected sewage thereby coming into contact with and saturating the compressed air/contaminated air being conveyed to the mixing device.
The invention further provides apparatus for the generation of electricity and the complementary purification of sewage comprising in combination:
(I) a heating vessel;
(ii) means for introducing a stream of treated sewage to the heating vessel;
(iii) a gas turbine comprising a compression stage, for compression of air or air and contaminated air, and an expansion stage which drives the compression stage and an alternator to generate electricity;
(iv) means for conveying the compressed air or compressed air and contaminated air through a pressure vessel, wherein it is saturated, to a mixing device where the compressed air/contaminated air is mixed with a separately introduced stream of fuel gas;
(v) a combustion chamber wherein the mixture of compressed air/contaminated air and fuel gas is combusted and from whence the hot resultant gases are conveyed to the expansion stage of the gas turbine;
(vi) means for heating the treated sewage in the heating vessel to disinfect the heated sewage, the heating means comprises the exhaust gases from the expansion stage of the gas turbine;
(vii) means for introducing the hot disinfected sewage into the compressed air/contaminated air conveying means, the disinfected sewage thereby coming into contact with and saturating the compressed air/ contaminated air being conveyed to the mixing device.
The invention will now be described with sequential reference to (I) preferred sub-generic features, (ii) a specified embodiment itself described with reference to the accompanying schematic drawing, and (iii) a working example (likewise described in conjunction with and reference to the illustrated drawing). The various integer(s) constituting the preferred embodiments of the several means referred to above will be readily identifiable in the detailed description and drawing. It is to be understood that, being in respect of preferred/illustrative features, this description should not be limitatively construed.
For convenience, the expression xe2x80x9ccompressed air or compressed air and contaminated airxe2x80x9d may frequently be simply expressed as xe2x80x9ccompressed air/contaminated airxe2x80x9d.
In preferred forms of the invention:
The temperature to which the sewage is heatedxe2x80x94to ensure disinfection of the sewage and destruction of resident pathogens, colliforms and the likexe2x80x94is generally in the range 100-180xc2x0 C., more preferably 130-150xc2x0 C. The residence time at the operating temperature is typically in excess of five minutes.
The turbine is a recuperated gas turbine.
The mixture of compressed air/contaminated air and fuel gas is combusted in a combusted of the type described in co-pending application PCT/AU95/00719.
The contaminated air introduced into the compressor of the turbine is comprised of foul air and methane gas from the sewage treatment plant (the foul air/methane being thereby incinerated with distribution of the foul odours and utilization of the energy value of the methane).
The hot disinfected sewage is introduced into the pressure vessel in which it comes into contact with the compressed air/contaminated air passing through the vessel to the mixing device, by means of a spray system/liquid distribution device.
The pressure vessel is also equipped with gas/liquid contacting mist eliminator systems through which the saturated stream of compressed/contaminated air passes en route to the mixing device.
a portion of the stream of treated sewage passing from the sewage treatment plant to the heating vessel is diverted to a cooling coil in the pressure vessel and thence back to the stream.
The fuel gas may be derived from coal or other solid carbonaceous fuel gasification as described in co-pending application PCT/AU96/00483.